With growing awareness of the capacity of chemicals to adversely affect male reproduction, there is a pressing need to increase our knowledge about the mechanisms by which chemicals cause reproductive damage. The need is apparent since the ability both to extrapolate from laboratory animals to man and to improve toxicity testing depends on knowledge of the biological system and how toxicants perturb it. In the proposed studies, the class of testicular toxicants of interest are the nitroaromatics which are widely used as pesticides, explosives, pharmaceuticals and chemical intermediates in industrial syntheses. The model testicular toxicant and nitroaromatic is 1,3-dinitrobenzene (1,3-DNB). Using laboratory animal models, the strategy will be first to investigate the role of metabolism (both hepatic and intratesticular) in the testicular toxicity of 1,3-DNB. Which metabolite is responsible for toxicity and the nature of its interactions with cellular constituents will represent the second stage of the research. Taking into account the information derived from the animal studies, a final goal is to establish relationship between the animal model and the human situation using human tissue in in vitro studies. To accomplish these objectives, the approach will be to test hypotheses in vivo then investigate specific cellular events in vitro. The first specific aim is to establish what role the liver plays in the modulation of 1,3-DNB testicular toxicity. Preliminary studies have implicated extratesticular events in 1,3-DNB toxicity since intratesticular administration of high levels of 1,3-DNB did not result in significant testicular damage. Two scenarios could be envisioned which could account for this result. Firstly, the liver may metabolize 1,3-DNB to a species which is capable of circulating in the blood and eliciting toxicity in the testes. Nitronitrosobenzene will be investigated as the most likely candidate for a circulating toxic metabolite. Second, a less direct interaction can be proposed where liver metabolism generates methemoglobin-forming metabolite(s) setting up conditions of oxygen deficiency. Low oxygen conditions within the testes would promote reductive metabolism of 1,3-DNB either magnifying the oxygen deficit through an increase in oxygen utilization via redox cycling or increasing the formation of a toxic metabolite such as nitronitrosobenzene. Magnification of the oxygen deficiency could lead to disruption of Sertoli cell homeostasis with a consequent inability to support the developing germ cell population. For specific aim two, in vitro mechanistic studies will explore further the in vivo hypotheses focussing on 1) detection of cellular indicators of metabolic activation to metabolite(s) capable of redox cycling, and 2) the capacity of the electrophilic metabolite nitronitrosobenzene to bind to cellular nucleophiles and disrupt cellular homeostasis. Also; the subcellular localization and identity of the reductases involved in 1,3-DNB metabolism will be investigated. The third specific aim is to compare the capacity of human and animal tissue to metabolize 1,3-DNB and to predict relative toxicity between species.